In many sensor applications, the data collected from individual nodes is aggregated at a base station or host computer. To reduce energy consumption, many systems also perform in-network aggregation of sensor data at intermediate nodes enroute to the base station. Most existing aggregation algorithms and systems do not include any provisions for security, and consequently these systems are vulnerable to a wide variety of attacks. In particular, compromised nodes can be used to inject false data that leads to incorrect aggregates being computed at the base station. We discuss the security vulnerabilities of data aggregation systems, and present a survey of robust and secure aggregation protocols that are resilient to false data injection attacks.